Probably money, or the lack of it. Anyone with any talent now immediately ships out, often before most Swedes are even aware they have a decent player on their hands. No player development really takes place.
In the the 90s, the core of that Swedish had longer stints a Swedish clubs when the financial playing of was a bit more even - when clubs like IFK Gothenburg could win European competitions or beat the likes of Bayern Munich.
Shoddy coaching doesn’t help either, hence the success of unknowns like Graham Potter.

I always thought he was an asshole. Like the time he ran the length of the pitch to tell his own fans to shove it after being criticised for whatever it was. Money grabbing, I think. Then went and left for West Ham after claiming he wanted Champions League football....